The overall goal of this research is the delineation of biochemical pathways which operate in the process initiated by the binding of an agonist to surface receptors, and leading to platelet secretion, activation of fibrinogen binding sites and aggregation. As part of our ongoing studies in this line of investigation, we have prepared and isolated a stimulatory monoclonal antibody (M.Ab.), named F- 11. M.Ab. F-11 acts as an agonist which induces secretion and aggregation of human platelets. The clinical significance of antibodies which activate platelets is well documented, but detailed characterization of the surface antigens that serve as receptors in this process and the biochemical pathways triggered by such antibodies has not yet been delineated. We expect that detailed studies of M.Ab. F-11 will begin to fill these gaps. The use of M.Ab. F-11 provides two distinct advantages: 1) The protein recognized by M.Ab. F-11 on the platelet surface has been identified and partially characterized in our laboratory. Thus, our studies focus now on an activation process initiated by a receptor whose molecular entity is already known. 2) The activation of human platelets by purified IgG of M.Ab. F-11 involves a lag period of 6 to 20 minutes which is dependent on M.Ab. F-11 concentration. This latency period permits detailed measurements of the sequence of molecular events leading to platelet secretion and aggregation. Accordingly, the Research Plan submitted here is designed to achieve the following specific goals: 1) Purification of a non-denatured form of the platelet surface protein recognized by M.Ab. F-11; 2) Characterization of the F-11 antigen and investigation as to whether it is a unique receptor, part of the receptor complex for one of the naturally- occurring platelet agonists, or a component of a known signal transduction system, 3) Determination of the biochemical pathway(s) operating in the activation of platelets by F-11; and, 4) Development of polyclonal and monoclonal antibodies to the purified F-11 receptor, for use in cellular localization, tissue distribution, and for immunological mapping of functional domains of the platelet F-11 antigen. We expect that accomplishment of these research goals will provide new and significant information on basic mechanisms operating during platelet activation. The health-related implications of these studies are particularly relevant to pathophysiological states involving interaction of antibodies with circulating platelets.